1. Field of the Invention
The present invention relates to distributing power to electronic components on a printed circuit board.
2. Background of the Related Art
Many commercially produced electronic devices include printed circuit board assemblies. A printed circuit board (PCB) comprises a non-conductive substrate that supports electronic components that are interconnected using conductive pathways etched from copper sheets laminated onto the PCB substrate. Electronic components on a PCB that require a large amount of current may have a conductive pathway with an increased number of layers, a heavier copper weight, a decreased pathway length, an increased pathway width or some combination of these features to conduct the required current to the electronic component without excessive heat caused by electrical resistance. However, due to staggering increases in the cost of copper, the cost of a PCB increases substantially if the PCB includes power distribution pathways with more layers, greater copper weight or increased pathway width in order to provide the required current to the electronic components.
An internal plane is a conductive layer disposed within the PCB substrate to carry current to vias formed in the PCB. Vias are plated through-holes in the PCB that are used to connect conductive pathways or electronic components on either side of the substrate to current-carrying internal plains. Power planes, ground planes, and the vias used to supply current from an internal power plane within the substrate to conductive pathways or electronic components on the sides of the PCB make up a large portion of the copper used in a PCB. As sophisticated electronic components increase the demand for power in the PCB, the population of vias and pathways on a PCB causes increased resistance to current flow and the generation of an excessive amount of heat. This increased amount of electrical resistance impairs the voltage available to the electronic components through the internal planes, vias and pathways on the PCB.
Where the cost of conductive material (copper) and/or the resistance to current flow become excessive, bus bars have been utilized to deliver sufficient power to current-consuming electronic components on the PCB. A bus bar is an electrically-conductive member that receives electrical current from a current source and distributes the current to the plurality of electronic components on the PCB. A bus bar is generally larger than an internal plane and takes up real estate on the PCB, but imposes considerably less resistance to current flow than do thin internal plains disposed in layers within the PCB substrate.
However, interfacing a bus bar to a PCB board is challenging. Soldering the bus bar to the PCB is impractical because a large amount of heat is needed to melt the solder to provide a conductive contact, and the heat produced by a soldering iron (or a soldering “gun”) may cause thermal damage to sensitive electronic components on the PCB. The obstacles to soldering are exacerbated by the high thermal conductivity of the bus bar, which causes a large portion of the heat provided to a solder point to be dissipated. As a result, an even greater amount of heat is required in order to melt the solder. Even if a suitable connection between the bus bar and the PCB can be made, the differences in the thermal expansion coefficients of the pathway material (copper), the solder material (tin and sometimes lead) and the bus bar material (copper and aluminum) will undergo different amounts of thermal expansion. Cyclic heating and cooling of the connection, due to variations in the current flow through the connection, promotes cyclic stress cracking and ultimately failure of the electrical contact.
Alternatively, a bus bar may be interfaced with a power sub-domain on a PCB using a screw terminal with a screw threadably received in a base to capture an extension from the bus bar against an extension from the PCB power sub-domain between the base and a head on the screw, and to hold these extensions in conductive contact one with the other. However, screw terminals may be compromised by vibrations and/or by the cyclic thermal stresses that cause the screw to gradually back out from the base or the extensions and to become dislodged from firm conductive contact one with the other. Also, screw terminals take up precious space on the PCB.
For optimal PCB management, it is useful to determine the rate of power consumption by each power sub-domain, each electronic component or each grouping of electronic components on a PCB. Current sense elements are devices that can be installed on discrete conductive pathways on a PCB to measure the current flowing to a power sub-domain or through an electrical contact of interest. Unfortunately, conventional current sense elements consume valuable space on the PCB and impose resistance to current flow through the contact that is being monitored by the current sense element. The result is unwanted heat and increased resistance of adjacent components. Low-resistance (less than 0.001 ohm) current sense elements may be used, but these devices are very costly and typically exhibit unacceptably large tolerances that may lead to inaccurate current measurements.